1. Field of the Invention
The invention relates to a carrier and a method of manufacturing the carrier and, more particularly, to a carrier for rotatably supporting rotating bodies such as gears and pulleys and a method of manufacturing the carrier.
2. Description of Related Art
A planetary gear unit is generally employed in an automatic transmission of an automobile. The planetary gear unit has a sun gear, a ring gear disposed around the sun gear, a planetary gear or a pinion (hereinafter referred to generically as a planetary gear) disposed between the sun gear and the ring gear so as to engage them, and a carrier for rotatably supporting the planetary gear.
As an example of a carrier for supporting a planetary gear or the like of a planetary gear unit, Japanese Patent Application Laid-Open No. HEI 10-288248 discloses a planetary gear unit for an automatic transmission according to the related art of the invention. This planetary gear unit has a ring gear, a sun gear and a pinion that engage one another, and a carrier for rotatably supporting the pinion. The carrier is composed of a boss connected to a shaft of the automatic transmission, a radially extended plate, a carrier plate formed of a salient axially protruding from the radially extended plate, and a base plate having holes into which the salient of the carrier plate is fitted. The salient of the carrier plate is made thicker than the boss and the radially extended plate by differential-thickness press working.
In the planetary gear unit disclosed in the aforementioned publication, as shown in FIG. 39, a carrier C′ is composed of a carrier plate 31 spline-connected to a shaft of an automatic transmission and a disc-shaped base plate 32. The carrier plate 31 is composed of a disc-shaped plate 31a and a plurality of columns (salients) 33 protruding towards the base plate 32 from the outer periphery of the plate 31a in a direction substantially parallel to the shaft. Fitting portions 33a with a reduced thickness are formed at the ends of the columns 33. The base plate 32 is constructed of an annular sheet material having at its center a through-hole 32a through which a shaft (not shown) or a sun gear (not shown) is inserted. Fitting holes 32b, into which the fitting portions 33a formed at the ends of the columns 33 of the carrier plate 31 are fitted, are formed on the outer circumferential side of the base plate 32. The carrier plate 31 and the base plate 32 are connected by fitting the fitting portions 33a formed at the ends of the columns 33 into the fitting holes 32b and welding parts of the fitting portions 33a fitted into the fitting holes 32b to the base plate 32. That is, according to this carrier, the carrier plate 31 and the base plate 32 are formed individually, assembled, welded, and then integrated. The carrier plate 31 and the base plate 32 are obtained from press-worked sheet materials. The aforementioned publication also discloses that the carrier plate and the base plate can be formed by forging or forging+cutting and the like instead of press working. In such a carrier, the pinion is inserted into the carrier among the columns 33 and between the carrier plate 31 and the base plate 32, and openings 35 defining a space for engagement of tooth tops of the pinion gear with the ring gear are made.
According to another related art, the base plate 32 and the carrier plate 31 having the columns 33 as in the carrier of the aforementioned related art are formed by sintering, assembled, connected by waxing instead of welding, and then integrated into a carrier.
In addition, according to still another related art, the carrier plate 31, the base plate 32 and the columns 33 are cast and integrated into a carrier. In this related art, in order to ensure that opposed faces of the carrier plate 31 and the base plate 32 are parallel to each other, the carrier plate 31 and the base plate 32 are cut after the carrier has been formed integrally. However, among the aforementioned related arts, as for the carrier disclosed in Japanese Patent Application Laid-Open No. HEI 10-288248, as described above, every time a carrier is manufactured, the carrier plate 31 and the base plate 32 that have been formed individually need to be assembled, connected by welding or waxing, and then integrated. For this reason, the number of manufacturing processes is great and each process requires its own plant, which causes a problem of the inability to reduce the cost.
Furthermore, for the purpose of rotatably supporting rotating bodies, the carrier is required to guarantee a high degree of parallel precision of the opposed faces of the carrier plate 31 and the base plate 32. However, among the aforementioned related arts, as for the art wherein the carrier plate 31 and the base plate 32 that have been formed individually by press working or sintering are assembled by welding or waxing, the parallel precision of the opposed faces cannot be enhanced due to an error caused during an assembling operation, a tolerance of pressing during the manufacture of the carrier plate 31 and the base plate 32, a distortion caused during sintering, welding heat at the time of connection, or a distortion caused by waxing. In order to solve this problem, it can be considered to dispose a mandrel having a predetermined width between the opposed faces of the carrier plate 31 and the base plate 32 after the carrier plate 31 and the base plate 32 have been integrated, and swage them towards the mandrel. However, in the aforementioned related art, since a welded portion or a waxed portion for connecting the carrier plate 31 and the base plate 32 cracks, it is impossible to perform swaging.
Further, among the aforementioned related arts, in the case where the carrier plate 31 and the base plate 32 that have been formed individually are assembled by welding, since it is necessary to prevent the fitting portions 33a of the columns 33 and the base plate 32 having the fitting holes 32b from being melted down during welding and guarantee sufficient rigidity, it is impossible to thin at least the base plate 32 and the columns 33 or narrow the peripheries of the columns 33. This makes it impossible to save the weight of the carrier.
In addition, in the case where the columns 33 of the carrier plate 31 and the base plate 32, which have been pressed, are assembled by welding, since the carrier plate 31 and the base plate 32 have been formed individually, the interrupted flow of a material causes a problem of low rigidity. In the case where the columns 33 of the carrier plate 31 and the base plate 32, which have been formed by sintering, are assembled by welding, since powder materials with a low density are used and there is no material flow, there is caused a problem of much lower rigidity.
Among the aforementioned related arts, as for the carrier into which the carrier plate, the base plate and the joints are cast and integrated, it is necessary to cut the carrier plate and the base plate after the carrier has been formed integrally. For this reason, the number of manufacturing processes increases and materials are wasted, which causes a problem of an increase of the cost. Because the carrier that has been formed by casting is obtained simply by forming a molten material and a density of contents of the carrier is low and there is no material flow, there is caused a problem of even lower rigidity.
Because of the problem of low rigidity as described above, namely, the problem of low rigidity per unit weight, the carrier that has been manufactured by sintering or forging needs to be made thick so as to guarantee predetermined rigidity. This causes problems of more wasted materials and the inability to save the weight of the carrier.